In one aspect, this invention relates to reagents that can be used to modify biomaterial surfaces or to fabricate new biomaterials. In another aspect, the invention relates to biomaterials having surfaces that have been prepared or modified to provide desired bioactive function.
Biomaterials have long been used to fabricate biomedical devices for use in both in vitro and in vivo applications. A variety of biomaterials can be used for the fabrication of such devices, including ceramics, metals, polymers, and combinations thereof. Historically, such biomaterials were considered suitable for use in fabricating biomedical devices if they provided a suitable combination of such basic properties as inertness, low toxicity, and the ability to be fabricated into desired devices. (Hanker, J. S. and B. L. Giammara, Science 242:885-892, 1988).
As the result of more recent advances, devices can now be provided with surfaces having various desirable characteristics, e.g., in order to better interface with surrounding tissue or solutions. For instance, approaches have been developed to promote the attachment of specific cells or molecules to device surfaces. A device surface, for instance, can be provided with a bioactive group that is capable of attracting and/or attaching to various molecules or cells. Examples of such bioactive groups include antigens for binding to antibodies, ligands for binding to cell surface receptors, and enzyme substrates for binding to enzymes.
Such bioactive groups have been provided on the surfaces of biomaterials in a variety of ways. In one approach, biomaterials can be fabricated from molecules that themselves present the desired bioactive groups on the surfaces of devices after fabrication. However, desirable bioactive groups are typically hydrophilic and cannot be incorporated into most metals or hydrophobic polymeric biomaterials at effective concentrations without disrupting the structural integrity of such biomaterials.
An alternative approach involves adding bioactive groups to the surfaces of biomaterials, e.g., after they have been fabricated into medical devices. Such bioactive groups can occasionally be added by adsorption. However, groups that have been added by adsorption cannot typically be retained on surfaces at high levels or for long periods of time.
The retention of such bioactive groups on a surface can be improved by covalent bonding of those groups to the surface. For instance, U.S. Pat. Nos. 4,722,906, 4,979,959, 4,973,493 and 5,263,992 relate to devices having biocompatible agents covalently bound via a photoreactive group and a chemical linking moiety to the biomaterial surface. U.S. Pat. Nos. 5,258,041 and 5,217,492 relate to the attachment of biomolecules to a surface through the use of long chain chemical spacers. U.S. Pat. Nos. 5,002,582 and 5,263,992 relate to the preparation and use of polymeric surfaces, wherein polymeric agents providing desirable properties are covalently bound via a photoreactive moiety to the surface. In particular, the polymers themselves exhibit the desired characteristics, and in the preferred embodiment, are substantially free of other (e.g., bioactive) groups.
Others have used photochemistry to modify the surfaces of biomedical devices, e.g., to coat vascular grafts. (See, e.g., Kito, H. et. al., ASAIO Journal 39:M506-M511, 1993. See also Clapper, D. L., et. al., Trans. Soc. Biomat. 16:42, 1993).
Cholakis and Sefton synthesized a polymer having a polyvinyl alcohol (PVA) backbone and heparin bioactive groups. The polymer was coupled to polyethylene tubing via nonlatent reactive chemistry, and the resultant surface was evaluated for thromboresistance in a series of in vitro and in vivo assays. For whatever reason, the heparin in the polymer prepared by Cholakis and Sefton did not provide effective activity. (Cholakis, C. H. and M. V. Sefton, J. Biomed. Mater. Res. 23:399-415, 1989. See also Cholakis, C. H., et. al., J. Biomed. Mater. Res. 23:417-441, 1989).
Finally, Kinoshita et. al. disclose the use of reactive chemistry to generate polyacrylic acid backbones on porous polyethylene, with collagen molecules being subsequently coupled to carboxyl moieties on the polyacrylic acid backbones. (See Kinoshita, Y., et. al., Biomaterials 14:209-215, 1993).
Generally, the resultant coating in the above-captioned situations is provided in the form of bioactive groups covalently coupled to biomaterial surfaces by means of short linear spacers. This approach works well with large molecular weight bioactive groups, such as collagen and fibronectin, where the use of short spacers is desired and the size of the bioactive group is quite large compared to that of the spacer itself.
The approaches described above, however, with the possible exception of Kinoshita et al., are not optimal for coating small molecular weight bioactive groups. Kinoshita does appear to coat small molecular weight molecules, although it describes a laborious multistep process that can detrimentally affect both yield and reproducibility.
Small molecular weight bioactive groups are typically provided in the form of either small regions derived from much larger molecules (e.g., cell attachment peptides derived from fibronectin) or as small molecules that normally diffuse freely to produce their effects (e.g., antibiotics or growth factors). It appears that short spacers can unduly limit the freedom of movement of such small bioactive groups, and in turn, impair their activity when immobilized. What are clearly needed are methods and compositions for providing improved concentrations of bioactive groups, and particularly small molecular weight groups, to a biomaterial surface in a manner that permits improved freedom of movement of the bioactive groups.
The present invention addresses the needs described above by providing a xe2x80x9cpolybifunctionalxe2x80x9d reagent comprising a polymeric backbone bearing one or more pendent photoreactive moieties and one or more, and preferably two or more, pendent bioactive groups. The reagent preferably includes a high molecular weight polymer backbone, preferably linear, having attached thereto an optimal density of both bioactive groups and photoreactive moieties. The reagent permits useful densities of bioactive groups to be coupled to a biomaterial surface, via one or more photoreactive groups. The backbone, in turn, provides a spacer function of sufficient length to provide the bioactive groups with greater freedom of movement than that which could otherwise be achieved, e.g., by the use of individual spacers (as described above).
As an added advantage, the present reagent permits the formation of inter- and intra-molecular covalent bonds within and/or between polymer backbones and the biomaterial surface, thereby providing an optimal and controllable combination of such properties as coating density, freedom of movement, tenacity and stability.
In addition to its use in modifying a biomaterial surface, a reagent of the invention provides other benefits as well. The photoreactive moieties allow individual polymer molecules to couple efficiently (e.g.,.crosslink) with adjacent polymer molecules. This crosslinking characteristic allows the polymers to generate thick coatings upon biomaterial surfaces and/or to generate independent films and bulk materials, either in vitro or in vivo.
The present invention also discloses a method for synthesizing a polybifunctional reagent and for providing a coated surface, such as the surface of a biomaterial, or biomedical device fabricated from such a biomaterial. The coated surface, having molecules of the polybifunctional reagent attached thereto in order to provide the device with desirable properties or attributes.
The photoreactive moieties can be activated in order to attach the polybifunctional reagent to a surface providing abstractable hydrogen atoms in such a manner that the pendent bioactive group(s) retain their desired bioactive function. Preferably, the reagent is attached to the surface in a xe2x80x9cone stepxe2x80x9d method, that is, by applying a reagent to the surface and there activating one or more of its photoreactive groups in order to form a coating. In contrast, a xe2x80x9ctwo stepxe2x80x9d method would involve a first step of immobilizing a polymeric backbone via photochemical means, and a second step of attaching (e.g., thermochemically) one or more bioactive groups to the immobilized backbone.
Preferred polybifunctional reagents of the invention can be used to coat the surfaces of existing biomaterials and/or to generate new biomaterials, e.g., by the formation of bulk materials. In either case, they can improve the surface properties of a biomedical device by providing covalently bound bioactive groups at the device surface. Preferred bioactive groups, in turn, act by either noncovalently binding to, or acting upon, specific complimentary portions of molecules or cells that come into contact with such groups.
In one preferred embodiment, a polybifunctional reagent of the invention is synthesized having a polymeric backbone, one or more photoreactive moieties, and two or more bioactive groups. The polymeric molecule of the invention is brought into contact with the surface of a previously formed biomaterial or into contact with another polymeric molecule of the invention. The photoreactive moieties are energized via an external stimulation to form, by means of active specie generation, a covalent bond between the reagent molecule and either the biomaterial surface or another reagent molecule. For instance, a biomaterial can be wetted in a solution containing a suitable reagent (typically for 0.1-5 minutes) and then exposed to light (typically for 0.1-2 minutes) to achieve covalent coupling.
Preferred bioactive groups function by promoting the attachment of specific molecules or cells to the surface. Preferred bioactive groups include, but are not limited to, proteins, peptides, carbohydrates, nucleic acids and other molecules that are capable of binding noncovalently to specific and complimentary portions of molecules or cells. Examples of such specific binding include cell surface receptors binding to ligands, antigens binding to antibodies, and enzyme substrates binding to enzymes. Preferably, the polymeric backbone comprises a synthetic polymeric backbone selected from the group consisting of addition type polymers, such as the vinyl polymers. More preferably, the photogroups each comprise a reversibly photoactivatable ketone.
As used herein, the following terms and words will have the following ascribed meanings:
xe2x80x9cbiomaterialxe2x80x9d will refer to a material that is substantially insoluble in aqueous systems and that provides one or more surfaces for contact with fluids containing biological molecules, e.g., in vivo or in vitro aqueous systems containing tissues, cells, or biomolecules;
xe2x80x9cdevicexe2x80x9d will refer to a functional object fabricated from a biomaterial;
xe2x80x9ccoatingxe2x80x9d, when used as a noun, will refer to one or more polymer layers on a biomaterial surface, and in particular, to one or more layers immobilized on a biomaterial surface by the activation of a polybifunctional reagent of the present invention;
xe2x80x9cpolybifunctional reagentxe2x80x9d, when used in the context of the presently claimed reagent, will refer to a molecule comprising a polymer backbone, to which are covalently bonded one or more photoreactive moieties and two or more bioactive groups;
xe2x80x9ca photoreactive moietyxe2x80x9d will refer to a chemical group that responds to a specific applied external energy source in order to undergo active specie generation, resulting in covalent bonding to an adjacent molecule or biomaterial surface;
xe2x80x9cbioactive groupxe2x80x9d will refer to a molecule having a desired specific biological activity, such as a binding or enzymatic (catalytic) activity;
xe2x80x9cpolymer backbonexe2x80x9d will refer to a natural polymer or a synthetic polymer, e.g., resulting from addition or condensation polymerization;
Preferred reagents of the invention comprise a synthetic polymer which serves as a backbone, one or more pendent photoreactive moieties which can be activated to provide covalent bonding to surfaces or adjacent polymer molecules, and two or more pendent low molecular weight biologically active moieties (bioactive groups).
Backbone. The polymer backbone can be either synthetic or naturally occurring, and is preferably a synthetic polymer selected from the group consisting of oligomers, homopolymers, and copolymers resulting from addition or condensation polymerization. Naturally occurring polymers, such as polysaccharides and polypeptides, can be used as well. Preferred backbones are biologically inert, in that they do not provide a biological function that is inconsistent with, or detrimental to, their use in the manner described.
Such polymer backbones can include acrylics such as those polymerized from hydroxyethyl acrylate, hydroxyethyl methacrylate, glyceryl acrylate, glyceryl methacrylate, acrylic acid, methacrylic acid, acrylamide and methacrylamide; vinyls such as polyvinyl pyrrolidone and polyvinyl alcohol; nylons such as polycaprolactam; derivatives of polylauryl lactam, polyhexamethylene adipamide and polyhexamethylene dodecanediamide, and polyurethanes; polyethers such as polyethylene oxide, polypropylene oxide, and polybutylene oxide; and biodegradable polymers such as polylactic acid, polyglycolic acid, polydioxanone, polyanhydrides, and polyorthoesters.
The polymeric backbone is chosen to provide a backbone capable of bearing one or more photoreactive moieties and two or more bioactive groups. The polymeric backbone is also selected to provide a spacer between the surface and the various photoreactive moieties and bioactive groups. In this manner, the reagent can be bonded to a surface or to an adjacent reagent molecule, to provide the bioactive groups with sufficient freedom of movement to demonstrate optimal activity. The polymer backbones are preferably water soluble, with polyacrylamide and polyvinylpyrrolidone being particularly preferred polymers.
Photoreactive moieties. Polybifunctional reagents of the invention carry one or more pendent latent reactive (preferably photoreactive) moieties covalently bonded to the polymer backbone. Photoreactive moieties are defined herein, and preferred moieties are sufficiently stable to be stored under conditions in which they retain such properties. See, e.g., U.S. Pat. No. 5,002,582, the disclosure of which is incorporated herein by reference. Latent reactive moieties can be chosen that are responsive to various portions of the electromagnetic spectrum, with those responsive to ultraviolet and visible portions of the spectrum (referred to herein as xe2x80x9cphotoreactivexe2x80x9d) being particularly preferred.
Photoreactive moieties respond to specific applied external stimuli to undergo active specie generation with resultant covalent boding to an adjacent chemical structure, e.g., as provided by the same or a different molecule. Photoreactive moieties are those groups of atoms in a molecule that retain their covalent bonds unchanged under conditions of storage but that, upon activation by an external energy source, form covalent bonds with other molecules.
The photoreactive moieties generate active species such as free radicals and particularly nitrenes, carbenes, and excited states of ketones upon absorption of external electric, electromagnetic or kinetic (thermal) energy. Photoreactive moieties may be chosen to be responsive to various portions of the electromagnetic spectrum, and photoreactive moieties that are responsive to e.g., ultraviolet and visible portions of the spectrum are preferred and are referred to herein occasionally as xe2x80x9cphotochemicalxe2x80x9d moiety.
Photoreactive aryl ketones are particularly preferred, such as acetophenone, benzophenone, anthraquinone, anthrone, and anthrone-like heterocycles (i.e., heterocyclic analogues of anthrone such as those having N, O, or S in the 10-position), or their substituted (e.g., ring substituted) derivatives. The functional groups of such ketones are preferred since they are readily capable of undergoing the activation/inactivation/reactivation cycle described herein. Benzophenone is a particularly preferred photoreactive moiety, since it is capable of photochemical excitation with the initial formation of an excited singlet state that undergoes intersystem crossing to the triplet state. The excited triplet state can insert into carbon-hydrogen bonds by abstraction of a hydrogen atom (from a support surface, for example), thus creating a radical pair. Subsequent collapse of the radical pair leads to formation of a new carbon-carbon bond. If a reactive bond (e.g., carbon-hydrogen) is not available for bonding, the ultraviolet light-induced excitation of the benzophenone group is reversible and the molecule returns to ground state energy level upon removal of the energy source. Photoactivatable aryl ketones such as benzophenone and acetophenone are of particular importance inasmuch as these groups are subject to multiple reactivation in water and hence provide increased coating efficiency. Hence, photoreactive aryl ketones are particularly preferred.
The azides constitute a preferred class of latent reactive moieties and include arylazides (C6R5N3) such as phenyl azide and particularly 4-fluoro-3-nitrophenyl azide, acyl azides (xe2x80x94COxe2x80x94N3) such as benzoyl azide and p-methylbenzoyl azide, azido formates (xe2x80x94Oxe2x80x94COxe2x80x94N3) such as ethyl azidoformate, phenyl azidoformate, sulfonyl azides (xe2x80x94SO2xe2x80x94N3) such as benzenesulfonyl azide, and phosphoryl azides (RO)2PON3 such as diphenyl phosphoryl azide and diethyl phosphoryl azide. Diazo compounds constitute another class of photoreactive moieties and include diazoalkanes (xe2x80x94CHN2) such as diazomethane and diphenyldiazomethane, diazoketones (xe2x80x94COxe2x80x94CHN2) such as diazoacetophenone and 1-trifluoromethyl-1-diazo-2-pentanone, diazoacetates (xe2x80x94Oxe2x80x94COxe2x80x94CHN2) such as t-butyl diazoacetate and phenyl diazoacetate, and beta-keto-alpha-diazoacetates (xe2x80x94COxe2x80x94CN2xe2x80x94COxe2x80x94Oxe2x80x94) such as t-butyl alpha diazoacetoacetate. Other photoreactive moieties include the aliphatic azo compounds such as azobisisobutyronitrile, the diazirines (xe2x80x94CHN2) such as 3-trifluoromethyl-3-phenyldiazirine, the ketenes (xe2x80x94CHxe2x95x90Cxe2x95x90O) such as ketene and diphenylketene.
Upon activation of the photoreactive moieties, the coating adhesion molecules are covalently bound to each other and/or to the material surface by covalent bonds through residues of the photoreactive groups. Exemplary photoreactive groups, and their residues upon activation, are shown as follows.
Bioactive Groups. Low molecular weight bioactive groups of the present invention are typically those that are intended to enhance or alter the function or performance of a particular biomedical device in a physiological environment. In a particularly preferred embodiment, the bioactive group is selected from the group consisting of cell attachment factors, growth factors, antithrombotic factors, binding receptors, ligands, enzymes, antibiotics, and nucleic acids. A reagent molecule of this invention includes at least one pendent bioactive group. The use of two or more pendent bioactive groups is presently preferred, however, since the presence of several such groups per reagent molecule tends to facilitate the use of such reagents.
Desirable cell attachment factors include attachment peptides (defined below), as well as large proteins or glycoproteins (typically 100-1000 kilodaltons in size) which in their native state can be firmly bound to a substrate or to an adjacent cell, bind to a specific cell surface receptor, and mechanically attach a cell to the substrate or to an adjacent cell. Naturally occurring attachment factors are primarily large molecular weight proteins, with molecular weights above 100,000 daltons.
Attachment factors bind to specific cell surface receptors, and mechanically attach cells to the substrate (referred to as xe2x80x9csubstrate adhesion moleculesxe2x80x9d herein) or to adjacent cells (referred to as xe2x80x9ccell-cell adhesion moleculesxe2x80x9d herein) [Alberts, B. et. al., Molecular Biology of the Cell, 2nd ed., Garland Publ., Inc., New York (1989)]. In addition to promoting cell attachment, each type of attachment factor can promote other cell responses, including cell migration and differentiation. Suitable attachment factors for the present invention include substrate adhesion molecules such as the proteins laminin, fibronectin, collagens, vitronectin, tenascin, fibrinogen, thrombospondin, osteopontin, von Willibrand Factor, and bone sialoprotein. Other suitable attachment factors include cell-cell adhesion molecules (xe2x80x9ccadherinsxe2x80x9d) such as N-cadherin and P-cadherin.
Attachment factors useful in this invention typically comprise amino acid sequences or functional analogues thereof that possess the biological activity of a specific domain of a native attachment factor, with the attachment peptide typically being about 3 to about 20 amino acids in length. Native cell attachment factors typically have one or more domains that bind to cell surface receptors and produce the cell attachment, migration, and differentiation activities of the parent molecules. These domains consist of specific amino acid sequences, several of which have been synthesized and reported. to promote the attachment, spreading and/or proliferation of cells. These domains and functional analogues of these domains are termed xe2x80x9cattachment peptidesxe2x80x9d.
Examples of attachment peptides from fibronectin include, but are not limited to, RGD (Arg Gly Asp (Referred to herein as SEQ ID NO:1)) [Kleinman, H. K, et. al., Vitamins and Hormones 47:161-186, 1993], REDV (Arg Glu Asp Val (Referred to herein as SEQ ID NO:2)) [Hubbell, J. A., et. al., Ann. N.Y. Acad. Sci. 665:253-258, 1992], and C/H-V (WQPPRARI or Trp Gln Pro Pro Arg Ala Arg Ile (Referred to herein as SEQ ID NO:3)) [Mooradian, D. L., et. al., Invest. Ophth. and Vis. Sci. 34:153-164, 1993].
Examples of attachment peptides from laminin include, but are not limited to, YIGSR (Tyr-Ile-Gly-Ser-Arg (Referred to herein as SEQ ID NO:4)) and SIKVAV (Ser-Ile-Lys-Val-Ala-Val (Referred to herein as SEQ ID NO:5)) [Kleinman, H. K, et. al., Vitamins and Hormones 47:161-186, 1993] and F-9 (RYVVLPRPVCFEKGMNYTVR or (Arg-Tyr-Val-Val-Leu-Pro-Arg-Pro-Val-Cys-Phe-Glu-Lys-Gly-Met-Asn-Tyr-Thr-Val-Arg) (Referred to herein as SEQ ID NO:6)) [Charonis, A. S., et. al., J. Cell Biol. 107:1253-1260, 1988].
Examples of attachment peptides from type IV collagen include, but are not limited to, HEP-III (GEFYFDLRLKGDK or Gly-Glu-Phe-Tyr-Phe-Asp-Leu-Arg-Leu-Lys-Gly-Asp-Lys (Referred to herein as SEQ ID NO:7)) [Koliakos, G. G, et. al., J. Biol. Chem. 264:2313-2323, 1989]. Desirably, attachment peptides used in this invention have between about 3 and about 30 amino acid residues in their amino acid sequences. Preferably, attachment peptides have not more than about 15 amino acid residues in their amino acid sequences.
Other desirable bioactive groups present in the invention include growth factors, such as fibroblastic growth factors, epidermal growth factor, platelet-derived growth factors, transforming growth factors, vascular endothelial growth factor, bone morphogenic proteins and other bone growth factors, neural growth factors, and the like.
Yet other desirable bioactive groups present in the invention include antithrombotic agents that inhibit thrombus formation or accumulation on blood contacting devices. Desirable antithrombotic agents include heparin and hirudin (which inhibit clotting cascade proteins such as thrombin) as well as lysine. Other desirable antithrombotic agents include prostaglandins such as PGI2, PGE1, and PGD2, which inhibit platelet adhesion and activation. Still other desirable antithrombotic agents include fibrinolytic enzymes such as streptokinase, urokinase, and plasminogen activator, which degrade fibrin clots. Another desirable bioactive group consists of lysine, which binds specifically to plasminogen, which in turn degrades fibrin clots.
Other desirable bioactive groups present in the invention include binding receptors, such as antibodies and antigens. Antibodies present on a biomaterial surface can bind to and remove specific antigens from aqueous media that comes into contact with the immobilized antibodies. Similarly, antigens present on a biomaterial surface can bind to and remove specific antibodies from aqueous media that comes into contact with the immobilized antigens.
Other desirable bioactive groups consist of receptors and their corresponding ligands. For example, avidin and streptavidin bind specifically to biotin, with avidin and streptavidin being receptors and biotin being a ligand. Similarly, fibroblastic growth factors and vascular endothelial growth factor bind with high affinity to heparin, and transforming growth factor beta and certain bone morphogenic proteins bind to type IV collagen. Also included are immunoglobulin specific binding proteins derived from bacterial sources, such as protein A and protein G, and synthetic analogues thereof
Yet other desirable bioactive groups present in the invention include enzymes that can bind to and catalyze specific changes in substrate molecules present in aqueous media that comes into contact with the immobilized enzymes. Other desirable bioactive groups consist of nucleic acid sequences (e.g., DNA, RNA, and cDNA), which selectively bind complimentary nucleic acid sequences. Surfaces coated with specific nucleic acid sequences are used in diagnostic assays to identify the presence of complimentary nucleic acid sequences in test samples.
Still other desirable bioactive groups present in the invention include antibiotics that inhibit microbial growth on biomaterial surfaces. Certain desirable antibiotics may inhibit microbial growth by binding to specific components on bacteria. A particularly desirable class of antibiotics are the antibiotic peptides which seem to inhibit microbial growth by altering the permeability of the plasma membrane via mechanisms which, at least in part, may not involve specific complimentary ligand-receptor binding [Zazloff, M., Curr. Opinion Immunol. 4:3-7, 1992].
Biomaterials. Preferred biomaterials include those formed of synthetic polymers, including oligomers, homopolymers, and copolymers resulting from either addition or condensation polymerizations. Examples of suitable addition polymers include, but are not limited to, acrylics such as those polymerized from methyl acrylate, methyl methacrylate, hydroxyethyl methacrylate, hydroxyethyl acrylate, acrylic acid, methacrylic acid, glyceryl acrylate, glyceryl methacrylate, methacrylamide, and acrylamide; vinyls such as ethylene, propylene, styrene, vinyl chloride, vinyl acetate, vinyl pyrrolidone, and vinylidene difluoride. Examples of condensation polymers include, but are not limited to, nylons such as polycaprolactam, polylauryl lactam, polyhexamethylene adipamide, and polyhexamethylene dodecanediamide, and also polyurethanes, polycarbonates, polyamides, polysulfones, poly(ethylene terephthalate), polylactic acid, polyglycolic acid, polydimethylsiloxanes, and polyetheretherketone.
Certain natural materials are also suitable biomaterials, including human tissue such as bone, cartilage, skin and teeth; and other organic materials such as wood, cellulose, compressed carbon, and rubber.
Other suitable biomaterials are composed of substances that do not possess abstractable hydrogens to which the photogroups can form covalent bonds. One such class of biomaterials can be made suitable for coating via photochemistry by applying a suitable primer coating which bonds to the biomaterial surface and provides a suitable substrate for binding by the photogroups. A subset of this group includes metals and ceramics which have oxide groups on their surfaces and are made suitable for coupling via photochemistry by adding a primer coating that binds to the oxide groups and provides abstractable hydrogens. The metals include, but are not limited to, titanium, stainless steel, and cobalt chromium. The ceramics include, but are not limited to, silicon nitride, silicon carbide, zirconia, and alumina, as well as glass, silica, and sapphire. One suitable class of primers for metals and ceramics consists of organosilane reagents, which bond to the oxide surface and provide hydrocarbon groups (Brzoska, J. B., et. al., Langmuir 10:4367-4373, 1994). The investigators have also discovered that xe2x80x94SiH groups are suitable alternatives for bonding of photogroups.
A second class of biomaterials that require an organic primer are the noble metals, which include gold, silver, copper, and platinum. Functional groups with high affinity to noble metals include xe2x80x94CN, xe2x80x94SH, and xe2x80x94NH2, and organic reagents with these functional groups are used to apply organic monolayers onto such metals (Grabar, K. C., et. al., Anal. Chem. 67:735-743, 1995).
Another class of biomaterials that do not possess abstractable hydrogens are fibrous or porous. The invention polymers form covalently crosslinked polymer networks that fill the pores or form films around individual fibers and are therefore physically entrapped. Expanded polytetrafluoroethylene is such a biomaterial.
Biomaterials can be used to fabricate a number of devices capable of being coated with bioactive groups using a polybifunctional reagent of the present invention. Implant devices are one general class of suitable devices, and include, but are not limited to, vascular devices such as grafts, stents, catheters, valves, artificial hearts, and heart assist devices; orthopedic devices such as joint implants, fracture repair devices, and artificial tendons; dental devices such as dental implants and fracture repair devices; ophthalmic devices such as lenses and glaucoma drain shunts; and other catheters, synthetic prostheses and artificial organs. Other suitable biomedical devices include dialysis tubing and membranes, blood oxygenator tubing and membranes, blood bags, sutures, membranes, cell culture devices, chromatographic support materials, biosensors, and the like.
Preparation of Reagents. Those skilled in the art, given the present teaching, will appreciate the manner in which reagents of the present invention can be prepared using conventional techniques and materials. In one preferred method, a polymer backbone is prepared by the copolymerization of a base monomer, such as acrylamide or N-vinylpyrrolidone, with monomers having pendent photoreactive and/or thermochemically reactive groups. The polymers prepared by this copolymerization are then derivatized with the bioactive molecule by reaction through the thermochemically reactive groups. An example of such a coupling is the reaction between an N-oxysuccinimide (NOS) ester on the polymeric backbone with an amine group on the bioactive molecule.
An alternative preferred method involves the preparation of monomers that contain the desired bioactive group as well as a polymerizable function, such as a vinyl group. Such monomers can then be copolymerized with monomers containing photoreactive groups and with a base monomer such as acrylamide or N-vinylpyrrolidone.
A preferred procedure used to synthesize latent reactive peptide polymers involves the synthesis of N-substituted methacrylamide monomers containing each peptide (peptide monomer) and a methacrylamide monomer containing a substituted benzophenone (4-benzoylbenzoic acid, BBA). The peptide monomers were prepared by reacting the sulfhydryl moiety of each peptide with the maleimide moiety of N-[3-(6-maleimidylhexanamido)propyl]methacrylamide (Mal MAm). Then, each peptide monomer was copolymerized with acrylamide and the monomer containing BBA (BBA-APMA) to produce the final latent reactive peptide polymer.
Various parameters can be controlled to provide reagents having a desired ratio (whether on a molar or weight basis) of polymeric backbone, photoreactive moeities and bioactive groups. For instance, the backbone itself will typically provide between about 40 and about 400 carbon atoms per photoreactive group, and preferably between about 60 and about 300 carbon atoms.
With respect to the bioactive group, the length of the backbone can vary depending on such factors as the size of the bioactive group and the desired coating density. For instance, for relatively small bioactive groups (MW less than 3000) the polymeric backbone will typically be in the range of about 5 to about 200 carbon atoms per bioactive group, and preferably between about 10 and about 100. For larger bioactive groups, such as those having a molecular weight between about 3000 and about 50,000, the preferred backbone provides, on the average, between about 10 and about 5000 carbon atoms between bioactive group, and preferably between about 50 and 1000 carbon atoms. In each case, those skilled in the art, given the present description, will be able to determine the conditions suitable to provide an optimal combination of bioactive group density and freedom of movement.
Coating method. Reagents of the present invention can be coated onto biomaterial surfaces using techniques (e.g., dipping, spraying, brushing) within the skill of those in the relevant art. In a preferred embodiment, the polybifunctional reagent is first synthesized and then brought into contact (i.e., sufficient proximity to permit binding) with a previously formed biomaterial. The photoreactive group is energized via an external stimulation (e.g., exposure to a suitable light source) to form, via free active specie generation, a covalent bond between the reagent and either another polybifunctional reagent molecule or the biomaterial surface. This coating method is herein termed the xe2x80x9cone step coating methodxe2x80x9d, since photoreactive coupling chemistry attaches an invention polymer to a biomaterial surface, and no subsequent steps are required to add the bioactive group. The external stimulation that is employed desirably is electromagnetic radiation, and preferably is radiation in the ultraviolet, visible or infrared regions of the electromagnetic spectrum.
Photoactivatible polymers of the invention can also be used to immobilize biomoieties in patterns on the surfaces of biomaterials, for example using techniques previously described for generating patterns of coating with features of 50-350 xcexcm in size. (See, Matsuda, T. and T. Sugawara, J. Biomed. Mater. Res. 29:749-756 (1995)). For example, hydrophilic patterns that inhibit the attachment and growth of endothelial cells can be generated by: 1) synthesizing latent reactive hydrophilic polymers, 2) adding the latent reactive polymers to tissue culture polystyrene plates, 3) illuminating the polymers through a pattern photomask, and 4) removing nonimmobilized polymers by washing with an appropriate solvent.
Such an approach can be employed with polymers of the present invention in order to immobilize patterns of specific biomoieties. For example, microarrays of specific binding molecules (e.g., antibodies, antigens/haptens, nucleic acid probes, etc.) can be immobilized on optical, electrochemical or semiconductor sensor surfaces to provide simultaneous multianalyte assay capabilities or multiple sensitivity range assays for single analytes. Patterned immobilization also provides a useful tool for developing a xe2x80x9claboratory on a chip,xe2x80x9d in which sequential processing/reaction steps occur along a fluid movement path in a multistep microvolume assay system. Patterning of cell attachment factors, for instance, those that promote the attachment of neural cells to electrodes, will permit the development of: 1) new generations of ultrasensitive biosensors and 2) artificial limbs that are directly controlled by the patient""s nervous system.
Reagents of the invention can be covalently coupled to previously formed biomaterials to serve as surface coatings. The present reagent molecules can also be covalently coupled to adjacent molecules, in order to form films or bulk biomaterials. The surface coatings, films, and bulk biomaterials resulting from coupling via photoreactive moieties provide useful densities of bioactive groups on the surface of the resultant biomaterials.
Use of devices. Bioactive polymers of the present invention are used to modify the surfaces of existing biomaterials or to generate new biomaterials. Biomedical devices that contain the resultant biomaterials are used for a variety of in vitro and in vivo applications. For example, biomedical devices possessing cell attachment groups or growth factors as biomoieties promote the attachment and/or growth of cells on in vitro cell culture devices and improve tissue integration with implant devices such as vascular grafts, orthopedic implants, dental implants, cornea lenses, and breast implants. Biomedical devices possessing antithrombotic factors as biomoieties prevent thrombosis on the surfaces of blood contacting devices, such as catheters, heart valves, vascular stents, vascular grafts, stent grafts, artificial hearts, and blood oxygenators.
Biomedical devices such as resins or membranes possessing receptors or ligands as biomoieties can be used for affinity purification of a broad range of biomolecules. For example, heparin (which is also an antithrombotic moiety) is used to specifically bind and purify several clotting factors, protease inhibitors, lipoproteins, growth factors, lipolytic enzymes, extracellular matrix proteins and viral coat proteins. Staphylococcal Protein A specifically binds immunoglobulins and has proven to be very useful for purification of antibodies. Streptavidin is a protein that binds specifically to biotin with extremely high affinity. Streptavidin and biotin are a very useful pair of reagents as a secondary binding pair in diagnostic assays. Many times signal amplification, enhanced sensitivity and faster test performance can be achieved by using immobilized streptavidin.
Biomedical devices having surface-coated antibodies or antigens can be used in diagnostic tests that depend on the specificity of binding for sensitive detection of the complimentary antigen or antibody. The antibodies or antigens can be immobilized onto membranes, plastic tubes, microplate wells or solid state biosensor devices. Immobilized antibodies are also important for purification of a variety of biopharmaceutical agents. Proteins produced in bacteria or fungi by genetic engineering techniques can be purified by affinity purification with immobilized antibodies. Blood fractions, such as clotting factor VIII (antihemophiliac factor) are also purified by immobilized antibodies.
Biomedical devices having surfaces coated with nucleic acid sequences can be used to selectively bind complimentary nucleic acid sequences. Such devices are used in diagnostic assays to identify the presence of complimentary nucleic acid sequences in test samples. Devices having surface-coated enzymes as biomoieties can be used for a broad range of enzyme reactors, to catalyze either synthetic processes (e.g., making chiral pharmaceuticals) or degradative/conversion processes (e.g., degrading starch and converting glucose to fructose for making high fructose corn syrup).
Coated antimicrobial agents can be used to inhibit bacterial growth on the surfaces of devices. Such antimicrobial surfaces can reduce the rate of infections associated with implant devices, including several types of catheters (intravascular, peritoneal, hemodialysis, hydrocephalus, and urological), arteriovenous shunts, heart valves, vascular grafts, tracheotomy tubes, orthopedic and penile implants. Several in vitro devices can also benefit from such surfaces, e.g., by inhibiting biofilm formation. These include contact lens cases, dental unit water lines, plumbing used in food and pharmaceutical industries, food packaging, table tops and other surfaces used for food handling, and air filters.